(CNN) -- During much of Nelson Mandela's imprisonment, communication with him was limited. In the early days on Robben Island he could only receive one visitor and one letter every six months.

That improved over time, but his correspondence was still rationed. His former prison warder Christo Brandt remembers taking 12 birthday cards to Mandela on his 60th birthday. That was his correspondence quota at the time.

Besides being heavily censored, the lengths of his letters were carefully controlled by the apartheid prison authorities.

So Mandela drafted all his letters in a notebook first, carefully counted the number of words and edited the final version down to the required length.

The original drafts of all of his letters, meticulously written out in notebooks, are stored in the Nelson Mandela Foundation archives.